PROJECT SUMMARY/ABSTRACT: The human microbiota has co-evolved with the human host. Over the past decade, the scientific community has surveyed the composition of the these human microbial communities, sequenced the content of many human microbial genomes, identified associations between microbiome states and disease, and begun to uncover mechanisms by which these microbial cells function as a community and contribute to human health. However, we still have a limited understanding of how the human microbiome is passed from one generation to the next and what mechanisms govern the processes of infant microbiome assembly and early microbiome development. Prenatal and early-life environmental exposures, including those that impact microbiome development, appear to play important roles in subsequent metabolic programming and growth. The etiology of the underlying imbalance in energy homeostasis that leads to obesity is complex, resulting from interactions between individual behaviors, environmental factors and genetic susceptibility; and, we hypothesize that maternal microbiome dysbiosis and impairment in microbiome establishment contribute to obesity risk in offspring. In our Human Microbiome Project-funded initiatives, we previously studied how the microbiome impacts pregnancy and neonatal outcomes. We and others have identified vaginal microbiome profiles associated with health states and established that the vaginal microbiome changes in pregnancy with a shift towards lower diversity and Lactobacillus dominance; this shift likely represents the evolutionary role of preventing vertical ascension of microorganisms to the intrauterine environment. Our central hypothesis is that the composition of the maternal microbiome at birth is central to the passage of the microbiome to the neonate and impacts microbiome establishment of the child. In this pilot project, we pursue three Specific Aims by reassessing ~100 three-year-old children born vaginally at term and their mothers who participated in the Multi- Omic Microbiome Study-Pregnancy Initiative and enrolled in the Research Alliance for Microbiome Science Registry. In Aim 1, we will characterize the relations between the maternal vaginal and gut microbiome prior to delivery and the child's gut microbiome at age 3 years among mother-child pairs who experienced term vaginal delivery using metagenomics and 16S rRNA-based approaches. In Aim 2, we will assess whether intrapartum maternal antibiotic exposure modulates the relations between maternal vaginal-gut microbiome and the child's gut microbiome at age 3 years. In Aim 3, we will seek to identify candidate gut microbiome signatures that discriminate stool samples from children with overweight/obesity from matched healthy-weight controls and to explore associations between child weight status at age 3 years and maternal microbiome exposure at birth. These exploratory studies will provide a characterization of host-microbiome interactions from early pregnancy to early childhood and may lead to new interventions to modulate microbiome development.